The present invention relates to a method and apparatus for illuminating projecting features on the surface of a semiconductor wafer such as from light reflective bumps formed on the surface of the wafer. Depending on the manufacturing process, these bumps may, for example, be reflowed solder-tin-lead bumps, electroplated gold bumps or bumps of other materials which are used for making electrical contact to circuits included in a die containing the bumps.
According to at least one industry report, more than 2.3 billion flip chips will be produced annually by the year 2002 and, of these, sixteen percent will be processors or complex application specific integrated circuits with more than 400 bumped input/output connections per circuit. Microprocessors in production today have thousands of vapor-deposited applied or electroplated bumps, resulting in some cases in more than one-half million bumps per wafer. As bump pitch decreases and complex devices are produced on, for example, 300-mm diameter wafers, the total number of bumps per wafer is expected to exceed one million. By the bumping stage, such expensive wafer products will have acquired nearly their entire value, and their worth will depend upon how strictly the bumping process can be monitored and controlled.
Defective bumps typically must be identified before wafer probing. Bumps that are too large, small or missing altogether can pass through probing without incident and lead to device failures. Such failures waste the time spent testing the defective die and packaging expense. There is also always the chance that an integrated circuit with a bad bump that could have been discovered at the wafer stage will work through a final test and still fail in a customer system. Thus, wafer probing is not an entirely reliable screen for determining the existence of significant bump defects. Also, certain types of bump defects can ruin probe cards, which are typically very expensive, if such bumps are not detected before probing. For example, malformed bumps can bridge power and ground contacts, as can extraneous bump material, causing excessive current drop through a probe card. This also raises the possibility of damaging neighboring die if imperfect solder bumps in a die are probed at such high currents. Also, bumps that are too tall or irregular can bend probes. This can result in the costs associated with obtaining a new probe card, costs associated with tester downtime, and costs of labor required to replace the damaged card.
Manual inspection of wafers to identify defective bumps is a possibility. However, such an approach is tedious, slow, prone to error, and is therefore inefficient and undesirable for production volumes, especially where tightly packed bumps are included in complex circuit dies. Bumps inspection is desirable to, for example, determine the existence of missing bumps, bridged bumps, bumps which are too small, or large, bumps which are too tall or short, the presence of nodules extending from bumps, satellite or foreign material between the bumps, misplaced bumps, and contaminants on or between the bumps.
It has been discovered that, from the characteristics of rings of light reflected from bump surfaces, it is possible to evaluate the bumps to determine whether the bumps have desirable characteristics or are flawed.
The present invention relates to a method and apparatus for illuminating projecting features, such as bumps, on the surface of a semiconductor wafer so as to generate light patterns which may then be used to evaluate the features or bumps. The resulting reflected light patterns may be captured as pixel information and analyzed in any suitable way.
In accordance with one aspect of an embodiment of the present invention, an apparatus is provided for directing light toward at least one light reflective feature such as a bump formed on a wafer surface of a semiconductor wafer. The apparatus comprises a light source support which may take any suitable form. At least one first light source is carried by the light source support and is operable to direct a first ring pattern of light toward the bump on the first wafer surface. At least one second light source may also be provided. The second light source may also be carried by the light source support and is operable to direct a second ring pattern of light toward the bump. These ring patterns of light are then reflected directly from the bump. Alternatively, plural ring patterns of light may be reflected from the bump with at least one such pattern resulting from the direct incidence of light on the bump from a light source and a second such ring pattern resulting from the indirect reflection of light from the light source off the wafer surface.
In accordance with another aspect of an embodiment, the intensity of light from the first light source may be adjustable and the intensity of light from the second light source may also be adjustable. These light intensities are desirably independently adjustable.
As yet another aspect of an embodiment, the first wafer surface is positioned in a first wafer plane. In addition, the first light source directs a ring pattern of light toward the bump at a first angle of incidence relative to the first wafer plane. This first angle of incidence desirably is between about eighteen degrees and twenty-two degrees with twenty degrees being a specifically desirable example. Also, the second light source directs a ring pattern of light toward the bump at a second angle of incidence relative to the first wafer plane with the second angle of incidence being different from the first angle of incidence. Desirably, the second angle of incidence is between about fifty-eight degrees and about sixty-two degrees with sixty degrees being a specifically desirable example.
As another aspect of an embodiment, at least a third light source for directing a ring pattern of light toward the bump may be provided. The third light source may direct a ring pattern of light toward the bump at a third angle of incidence relative to the plane of the first wafer surface. The third angle of incidence is different from the first and second angles of incidence. Desirably, the third angle of incidence is between about forty-three degrees and about forty-seven degrees with forty-five degrees being a specifically desirable example.
Thus, the third light source is intermediate to the first and second light sources in that the angle of incidence of light from the third light source is between the angle of incidences of the first and second light sources in this specific embodiment.
Light from the three ring light sources may be selectively directed toward a bump. For example, the first and second ring light sources may simultaneously direct light toward the bump with the third ring light source being off under certain first wafer conditions. Alternately, the third ring light source may be operable to direct light toward the bump with the first and second ring light sources not directing light toward the bump, thus being considered off, under certain second wafer conditions. The first wafer conditions may comprise a relatively low reflective wafer surface which reduces the indirect or ghost reflection of light from the third ring light source from the wafer surface toward the bump if the third ring light source were on. The first wafer conditions may also comprise relatively tightly packed bumps on the wafer surface, meaning that the bumps are close enough to one another that adjoining bumps interfere with the indirect reflection of light from the third ring light source from the wafer surface to the bump. In contrast, the second wafer conditions may comprise a relatively high reflective wafer surface and relatively distantly spaced adjoining bumps. Under such conditions, light from the third ring light source may directly reach and reflect from the bump to provide a first ring pattern of light on the bump and also be reflected from the wafer surface to the bump to provide a second ring light pattern on the bump without substantial blockage by nearby bumps. The patterns on the bump being ring-like when the bump is well formed with deviations of the patterns from ring-like configurations indicating potentially problem bumps.
As another aspect of an embodiment, the intensity of light from all three of the first, second and third light sources may be variable and also may be independently variable relative to the intensity of light from the other ring light sources.
The light support for the ring light sources may be generally frustoconical in overall configuration. Alternatively, the light support may take any suitable shape. The planarity of the light support is desirably adjustable such that planes containing the first, second and third ring light sources may be positioned parallel to the first wafer surface, which is typically planar. In addition, the light support may comprise a first annular light supporting section for carrying the first light source and a second annular light supporting section for carrying the second light source. The third light source may also be carried by the second annular light supporting section. The second annular light supporting section is typically positioned above the first annular light supporting section when the lights are in use. The first and second annular light supporting sections are desirably interconnected. Gaps or apertures may be provided between the interconnections from the first to second annular light supporting sections. The first light source may comprise a plurality of discrete lighting elements distributed about the first annular light supporting section. In addition, the second light source may comprise a plurality of discrete lighting elements distributed about the second annular light supporting section. The third light source may also comprise a plurality of discrete lighting elements and may be distributed about the second annular light supporting section below the lighting elements of the second light source.
As a further aspect of an embodiment, the first light source may comprise a first set of a plurality of first light emitting diodes arranged in a ring to emit light in a first light source plane. In addition, the second light source may comprise a second set of a plurality of second light emitting diodes arranged in a ring to emit light in a second light source plane. Desirably, the first wafer surface of the semiconductor wafer is planar and in a plane which is parallel to the first and second light source planes. This embodiment may also include a third light source comprising a third set of a plurality of third light emitting diodes arranged in a third light source plane which is parallel to the first and second light source planes. The intensity of light from the first and second sets of light emitting diodes may be independently adjustable. In addition, the intensity of light from the third set of light emitting may be independently adjustable. The light emitting diodes may emit red light. Alternatively, the light emitting diodes may emit other colors of light with the light emitting diodes of each set typically being of the same color, which may be different from the color of the light emitting diodes of the other sets. Alternatively, in one embodiment at least a majority and desirably all of the light emitting diodes of the various sets may emit red light. The light emitting diodes may be of a type having a narrow focus such as no greater than about fifteen degrees.
The light support may comprise at least a first set of bores arranged in a first ring pattern with the first ring of light sources, such as the first set of light emitting diodes, each being positioned within a respective bore of the first set of bores. The light support may also comprise at least a second set of bores arranged in a second ring pattern. The second ring of light sources, such as the second set of light emitting diodes, may each be positioned within a respective bore of the second set of bores. In addition, the light support may comprise a third set of bores arranged in a third ring pattern. The third ring of light sources may comprise a third set of light emitting diodes which are each positioned within a respective bore of the third set of bores. The light emitting diodes may be press fit or otherwise held within the associated bores.
As yet another aspect of an embodiment, the first and second sets of light emitting diodes, and for that matter the third set of light emitting diodes, may each comprise plural light segments having a base with plural light emitting diodes carried by the base. The intensity of light from each light segment may be adjustable independently of the intensity of the light from the other light segments. As a specific example, eight light emitting diodes may be included in each segment.
The present invention is also directed toward methods of directing light toward at least one reflective bump or feature on a semiconductor wafer surface to form patterns of light on the bump or feature. In addition, the present invention is directed toward novel and non-obvious combinations and sub-combinations of elements used for illuminating projecting features on the surface of a semiconductor wafer such as light reflective bumps. In addition, the invention is directed toward novel and non-obvious method acts and steps alone, as well as in combination with one another, relating to illuminating projecting features on the surface of a semiconductor wafer. Such novel and non-obvious elements, steps and acts being set forth in the claims below.